Short version: This is my main Sonic fan character, Door Into Summer the Echidna. These are several drawings of her done over the years, put side by side to show how she has changed.

Really long version:
Door Into Summer was created in the sixth grade when I drew a picture of Julie-Su and thought it would be keen to alter and re-color it into a new character and name her Summer. "Summer" was the name of a Pomeranian that used to wander onto the school playground, so that may have been where I got the name. Unfortunately, I haven't been able to find any drawings of her from back then. She looked fairly pudgy (because of my drawing style back then) and had a round, hedgehog-like face with rounder eyes. Instead of having many shorter, purple-tipped locks, she only had three coming out of her forehead. She wore white mittens with purple cuffs and a rainbow-striped shirt with a V-neck. Her boots were similar to Amy Rose's but had a red, white, and blue color scheme.
This version of Summer was childlike, happy-go-lucky, and frequently love struck. She was fourteen years old and loved to dance.
At one point, I renamed her Hokusai, after a famous Japanese artist. Eventually I decided that was dumb and started calling her Summer again.
My common internet alias at the time was "Door Into Summer," after the music track in the game "Knuckles Chaotix." When people mistakenly assumed that that was also the name of my character, I decided that it really should be her name. She went by D.I.S. for short.

Cut to 2000. She looks very different. My changing drawing style made her go from chubby to anorexic. I belonged to a small, close-knit Sonic forum back then, and one of the members planned to use everyone's fan characters in his comic. His redesign of Summer deliberately made her clothes look like a Final Fantasy knockoff. He also made Summer a twenty-four year-old gambler. The comic never got off the ground, but I liked many of his ideas for Summer and made them part of the "official" Summer.
By this time, Summer was more aggressive, self-centered, slightly nutty, and somewhat shady. She could side with either the bad guys or the good guys.

Cut to 2001. Not much has changed, although this Summer is more calm and confident. And still a hard-core gambler, judging by the aces in her hand.

2002: Summer begins to get a little more conservative. Her garment is a long coat instead of a weird bejeweled cape thing with short sleeves. Her shirt has a more conservative cut. You can't see it behind her hands, but it no longer exposes her belly. She's still a bit cocky, but also more self-conscious, judging from her slight blush.

2003: A return to a soft, cuddly, slightly chubby style, kind of like the earliest Summer. Her personality is more benign and less aggressive. I decided that twenty-four was too old and I made her in her late teens, like me.

2005
Summer finally gets a much more "no-nonsense" wardrobe. Her personality continued to get more modest and less flamboyant, and I wanted her clothes to reflect this. I added a choker for no reason other than I like chokers.
Her expression is kind of ambiguous. It could be interpreted as snooty, but she's supposed to look serious, like she's considering something.

2007
Similar to the 2005 design, but modified to more closely resemble what I actually wear, or at least what I would like to wear. I added glasses because I wear glasses and they add character to the design.
This Summer's personality is kind of a mishmash of the previous personalities. She's fairly flawed and still retains a bit of darkness and nuttiness under the surface, but she's a kinder soul with a shy streak.
Her name is still Door Into Summer, but it's usually shortened to "Summer" instead of "D.I.S." because it rolls off the tongue easier and has pleasant connotations. Her legal surname is "McKay" (the name of her adoptive family) but she also uses "Echidna" as a more generic surname.